A permit from the US Fish and Wildlife Service is required to import an animal such as a black rhino into the country under the Endangered Species Act.

Humane Society president Wayne Pacelle writes in a blog post that killing one animal to save the species will goad other hunters to follow suit. "The first rule of protecting the rarest animals in the world is to protect each living member of that species," he writes.

"We plan to provide evidence to the Fish and Wildlife Service that trophy hunting a member of a critically endangered species is harmful to that species," writes Humane Society president Wayne Pacelle.

"The ESA makes it clear that such permits should only be granted when the import will enhance the survival of the species in the wild," he continues. "We plan to provide evidence to the FWS that trophy hunting a member of a critically endangered species is harmful to that species."

In response, the Humane Society has created a petition urging the Fish and Wildlife Service not to issue the import permit.

Knowlton is a hunting "consultant" for the Hunting Consortium Inc. who has appeared on the hunting show Jim Shockey's The Professionals. In recent days, he's emerged to give two interviews, first for local outlet WFAA, followed one day later by an interview on CNN, in which he reveals an increasing degree of anxiety.

On January 15, Knowlton told WFAA that the hunt was a challenge he welcomed. "I'm a hunter," he said. "I want to experience a black rhino. I want to be intimately involved with a black rhino."

By January 16, his tenor changed. CNN caught him holed up in a Las Vegas hotel room, surrounded by private security.

"If I sound emotional, it's because I have people threatening my kids," he said on CNN's Piers Morgan Live. "It's because I have people threatening to kill me right now [that] I'm having to talk to the FBI and have private security to keep my children from being skinned alive and shot at."

He told Morgan that he wants to keep the rhino's hide and donate the meat to communities in Namibia, but he doesn't know when he'll take his hunting expedition.

Knowlton also claimed that publicity scared away bidders, with a pool of 10 serious bidders dwindling to about three. "After what I'm going through now, I understand why they decided not to do it," he said.